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{{otheruses4|nuclear engineering|the topic in geology|Fissility (geology)|other uses|fission}}

In [[nuclear engineering]], a '''fissile''' material is one that is capable of sustaining a [[chain reaction]] of [[nuclear fission]]. 

All known fissile materials are capable of sustaining a chain reaction in which either thermal or [[slow neutron]]s  or [[fast neutron]]s predominate. That is, they can all be used to fuel:

* A [[thermal reactor]], with a [[neutron moderator]]
* A [[fast reactor]], with no moderator
* A [[nuclear explosive]]

==Fissile vs fissionable==
"Fissile" is distinguished from "'''fissionable'''".  "Fissionable" are any materials with atoms that can undergo nuclear fission.  "Fissile" is defined to be materials that are fissionable by [[neutron]]s with zero [[kinetic energy]].  "Fissile" thus, is more restrictive than "fissionable" &mdash; although all fissile materials are fissionable, not all fissionable materials are fissile.  Some authorities even restrict the term ''fissionable'' to mean only non-fissile materials.

Notably, [[uranium-238]] is fissionable but not fissile. Neutrons produced by fission of e.g. U-235 have an energy of ca. 1 MeV (100 TJ/kg, i.e. a speed of 14,000 km/s) and do not cause fission of U-238, but neutrons produced by [[deuterium]]-[[tritium]] fusion have an energy of 14.1 MeV neutrons (1400 TJ/kg, i.e. a speed of 52,000 km/s) and can easily fission uranium-238 and other non-fissile [[actinides]]. The neutrons produced by this fission are again not fast enough to produce new fissions, so U-238 does not sustain a chain reaction.

Fast fission of uranium-238 in the third stage of the [[fission-fusion-fission]] weapons contributes greatly to their yield and fallout. Fast fission of uranium-238 also makes a significant contribution to the power output of some [[fast breeder reactor]]s.

==Fissile nuclides==
Fissile [[nuclide]]s in [[nuclear fuel]]s include:

* [[Uranium-235]] which occurs in [[natural uranium]] and [[enriched uranium]]
* [[Plutonium-239]] bred from [[Uranium-238]] by [[neutron capture]]
* [[Plutonium-241]] bred from [[Plutonium-240]] by neutron capture
* [[Uranium-233]] bred from [[Thorium-232]] by neutron capture

In general, [[actinide]] isotopes with an odd number of neutrons are fissile. Most nuclear fuels have odd [[mass number|N]] (number of protons and neutrons) and even [[atomic number|Z]] (number of protons). Isotopes with an odd number of neutrons and odd number of protons (odd Z, even N) are shortlived because they can [[beta decay]] to an isotope with an even number of neutrons and even number of protons. (even Z, even N)

Fissile nuclides do not have a 100% chance of fissioning on absorption of a neutron. The chance is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the [[cross section]]s for fission and for capture emitting a [[gamma ray]], and the percentage of nonfissions are:
{|
!colspan=3|Thermal neutrons!! !!colspan=3|Epithermal neutrons
|-
!σ<sub>F!!σ<sub>γ!!%!! !!σ<sub>F!!σ<sub>γ!!%
|-
|585||99||14.5%||<sup>235</sup>U||275||140||34%
|-
|750||271||26.5%||<sup>239</sup>Pu||300||200||40%
|-
|1010||361||26.3%||<sup>241</sup>Pu||570||160||22%
|-
|531||46||8.0%||<sup>233</sup>U||760||140||16%
|}

<!-- These are fissionable, not fissile! They are not fissionable with slow neutrons. They possess a critical mass, but this is because of fast fission.

 Several other [[transuranic]] [[isotope]]s are known to be fissile, all of them having both even [[atomic number]]s and odd atomic [[mass number]]s. These include:

*[[Neptunium]]-237[http://packages.llnl.gov/doe_ord/o54803.html]
*[[Curium]]-244[http://packages.llnl.gov/doe_ord/o54803.html]
*[[Americium]]-241
-->
==Nuclear fuel==
To be a useful fuel for nuclear fission chain reactions, the material must:

* Be in the region of the [[binding energy]] curve where a fission chain reaction is possible (i.e. above [[radium]])
* Have a high probability of fission on [[neutron capture]]
* Release two or more neutrons on average per neutron capture (which means an even higher number on each fission, to compensate for nonfissions)
* Have a reasonably long [[half life]]
* Be available in suitable quantities

== Legal controls ==

The [[International Atomic Energy Agency]] used to categorize fissile materials according to their security requirements for transportation:<ref>Safe Transport of
Radioactive Materials, [[International Atomic Energy Agency]], 1964</ref><ref>10[[Code of Federal Regulations|CFR]]71, 49CFR173.403</ref>

* Fissile Class I:  no controls
* Fissile Class II:  limits on amount of materials shipped
* Fissile Class III: special shipping arrangements are needed

but these classes were replaced in the mid 1990s.<ref>[http://radlab.nl/radsafe/archives/9510/msg00203.html 49CFR & 10CFR71 changes]</ref>

== References ==
{{refs}}

== See also ==

* [[Fertile material]]
* [[Fission product]]
* [[Nuclear fusion]]
* [[Fissility]] (disambiguation)

[[Category:Nuclear physics]]
[[Category:Fissile materials| ]]

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